84’ Marathon stock motor filled with differential oil

[JB in PA]

First post here but have read many, thanks for the informative site and friendly community. I’m new to carts but not to building stuff. Went looking for a used electric cart to transform into a “go slow cart” as I promised my wife, for my kids. Ended up with an 84’ Marathon resistor cart that I knew was a total rust bucket for a couple hundred bucks. Figured I’d get enough to work with and do a ScottyB conversion to be able to control speed and have fun welding up the rest.
Looks like I only did half enough research before bringing this home as I’m not getting as much to work with as I had hoped but no big deal. I didn’t realize how limited this rear end/motor combo was until Scotty mentioned it but again not a huge deal if this is reliable,it won’t be a big wheeled working type cart. Everything is being replaced so figured 48v and Scotty conversion will work well.
Last step before ordering kit was verifying motor internals look OK. It tested “ok” with a small 12v battery (wheels spun, little sticky but sounded like brake rub). And continuity to frame tested ok.
Hoping to clean up brush dust or worst case have the stock motor rebuilt but was surprised to find an oil mess inside the motor this weekend. It looks to me like the seal between the rear differential and the motor shaft failed and all the oil was in the motor. It’s a black tar like mess everywhere. The rear is actually dry, it’s all in the motor .
Anyone ever seen this? My searches came up totally empty
Ok it won’t be fun but I can clean that up, other than the oil ,brushes and springs etc actually look good. But the rear differential has me worried.
There’s a strange rectangular hole in the differential near the input shaft. Is that supposed to be there? My first thought was I’m screwed, there’s a hole in the differential that let oil flood the input shaft area where the motor slides over, seal failed so into the motor. But where’s the shrapnel/pieces? It’s big enough I should see something. That hole isn’t supposed to be there is it?
Seal obviously has to be changed but what about this hole?
It’s at 9 to 10 o’clock looking into the input shaft.
My searches for a more modern rear diff have come up dry around here so hoping to work with this.
Is there some reason this hole is supposed to be here?

I’ll post pics when I can, just realized I have to be here a week first.

Any tips until pictures would be much appreciated!

[JB in PA]

Probably hard to tell what I'm talking about so here's some of the strange hole in the rear differential where it bolts up the motor. I circled it in red.
After thinking about it and staring at it I'm starting to think this is a normal hole and the spring loaded rubber dynamic seal simply failed and didn't keep the oil out of the motor. Seems like a lot to ask for that seal but it's been changed and motor is now cleaned up and quick test of motor tests OK so far.
Still a bit curious about that square hole letting that oil through, can anyone confirm out of curiosity if they've seen that before? I'm not heading for another oil bomb in the motor am I?